1. Field of the Invention
The present invention relates generally to digital air pressure gauges and more particularly to calibration methods for digital tire pressure gauges.
2. Description of the Prior Art
A digital pressure gauge generally involves the application of a microprocessor and a pressure measurement device such as a pressure sensor or load-cell coupled with a potentiometer. The microprocessor reads data out of the pressure measurement device and stores the data. The microprocessor then carries out control functions as well as math and data storage from programming. The microprocessor automatically compensates for errors and then displays a digital signal on the liquid crystal display (LCD) or light-emitting diodes (LED).
The most important thing about a digital pressure gauge is its accuracy. In pressure gauge design the input and output relationship must be predetermined, as without a defined reference relationship the measured pressure cannot be displayed. Currently, the best way to predefine the reference relationship is by setting the input and output data in a linear relationship.
There are many types of errors of the measurement device that can affect the accuracy of the digital gauge. They include span error, linearity error, and a random type offset error. FIGS. 1a–1c demonstrate graphically these unpredicted errors found within a typical tire pressure gauge measurement device; specifically, span error (FIG. 1a), non-linearity error (FIG. 1b) and random offset error (FIG. 1c). In each of these FIGS. 1a–1c, the predefined reference straight line 12 for the system establishes the relationship between input and output. In FIG. 1a, the measurement span error 16 shows that the span error is linear and increases with increases of input pressure. In FIG. 1b, the measured non-linear error 20 is dependent on the curvature between the reference line 12 and measurement line 20, and in FIG. 1c the random offset error 24 is not repeatable. These errors may be caused by the tolerances of mechanical parts, material property characteristics, or assembly mistakes, etc, and they cannot be predicted in advance. As long as these errors exist in the digital gauge, the input data and measured data will not have the same linear relationship as is predefined, and because of these errors the gauge will display an inaccurate pressure reading, so compensating for these errors is essential.
In current prior art devices, calibration is typically performed by applying the status equilibrium method. This involves predetermining a few reference pressure points to divide the pressure range into intervals. Using the digital gauge against a very accurate test gauge, calibration of the beginning and ending reference pressure point of each interval is done. The measured reference pressure voltage is stored in memory. When subsequent pressures are measured, the stored reference voltage is obtained from the memory, thereby attempting to give the correct voltage and compensating for errors. A linear interpolation method is used to calculate the value of pressure between reference pressure points. The drawback of this technique occurs if the measured voltage and the referenced voltage are not in a linear relationship, then use of the linear interpolation method to calculate voltage between reference pressure points will be inaccurate. This calibration technique is time-consuming and of high cost, and if a large number of pressure points are needed to compensate the nonlinear or random offset errors then this technique is not acceptable.